1. Field of the Invention
The present invention relates to a semiconductor accelerometer having a cantilevered beam formed on a semiconductor substrate and a method for producing the same.
2. Description of the Background Art
Recently, there has been developed a micro-miniature semiconductor accelerometer formed on a semiconductor substrate using thin film technology such as photolithographic technology similar to that employed in the fabrication of integrated circuits. Such a semiconductor accelerometer is designed to detect acceleration by sensing a resistance change due to the piezoresistance effect of a semiconductor film formed on a semiconductor substrate or a minute capacity change due to deflection of a cantilevered beam formed on the semiconductor substrate.
Since such a semiconductor accelerometer is formed using the thin film technology, as mentioned above, it has the excellent feature that it can be formed extremely small in size, for instance, with a length of approximately 100 .mu.m, a thickness of approximately 1 .mu.m, of a vibrating portion, and an overall chip size of approximately 1 mm square, and in addition, that it can be formed on the same substrate for integrated circuits with other elements.
One example of a conventional semiconductor accelerometer is described in "A Batch-Fabricated Silicon Accelerometer", IEEE Electron Devices, Vol. ED-26, No. 12, Dec. 1979, pp. 1911-1917, as shown in FIGS. 1a to 1c.
In the conventional semiconductor accelerometer, as shown in FIGS. 1a to 1c, an n-type silicon semiconductor substrate as a frame 21 is processed in a conventional manner to form a C-shape-like gap 24 in the middle portion to obtain a cantilevered beam 22 and a mass or weight 23, and a piezoresistor 25 of a diffused resistor is formed in the surface near the support portion of the cantilevered beam 22.
In this case, when an acceleration is applied to the accelerometer, the weight 23 is deflected to cause a distortion in the cantilevered beam 22. As a result, the piezoresistor 25 changes the resistance value due to the piezoresistance effect, and the resistance value change is detected to obtain the acceleration given to the accelerometer. Hence, high processing accuracy is required in formation of the cantilevered beam 22.
In FIGS. 2a to 2e, there is shown a method for producing the conventional semiconductor accelerometer shown in FIGS. 1a to 1c.
In FIG. 2a, after the formation of a diffused resistor or piezoresistor (not shown) in the upper surface of an n-type silicon semiconductor substrate 31 as a frame, upper and lower silicon oxide films 32 and 33 are formed on the upper and lower surfaces of the substrate 31 to cover the entire upper and lower surfaces. The lower silicon oxide film 33 is partially removed by photoetching to form opening portions 34 and 35 for forming a cantilevered beam portion and a lower gap, respectively.
In FIG. 2b, the anisotropic etching of the lower surface of the substrate 31 is carried out with the lower silicon oxide film 33 as the mask by using an etching solution including potassium hydroxide (KOH). In this case, the control of the thickness of the cantilevered beam portion is performed by regulating the temperature of the etching solution and the duration for etching.
In FIG. 2c, the upper silicon oxide film 32 is partially removed by the photoetching to form an opening portion 36 for forming an upper gap.
In FIG. 2d, the anisotropic etching of the substrate 31 is carried out using the upper and lower silicon oxide films 32 and 33 as the mask in the same manner as the step shown in FIG. 2b until a gap 38 penetrating the substrate 31 is formed.
In FIG. 2e, the upper and lower silicon oxide film 32 and 33 are removed by etching to form a cantilevered beam 37 and a weight 39, the gap 38 defining the external form of the cantilevered beam 37 and the weight 39.
In this method, it is very difficult to perform a precise control of the thickness of the cantilevered beam, and the thickness of the cantilevered beam varies widely.
In order to remove this problem, another method using an electrochemical etching stop technique for producing a semiconductor accelerometer has been proposed, as shown in FIGS. 3a to 3g.
In FIG. 3a, an n-type epitaxial layer 42 is formed on the upper surface of a p-type silicon semiconductor substrate 41, and an upper silicon oxide film 43 is formed on the epitaxial layer 42.
In FIG. 3b, a p-type diffusion region 44 is formed in the epitaxial layer 42 by doping an impurity.
In FIG. 3c, the upper silicon oxide film 43 is partially removed in the same manner as described above to form an opening portion 45 for forming an n-type silicon contact portion.
In FIG. 3d, an electrode 46 is formed over the entire upper surface of the resulted substrate.
In FIG. 3e, a lower silicon oxide film 47 to be used as a mask for etching is formed on the lower surface of the substrate 41, and the lower silicon oxide film 47 is partially removed in the same manner as described above to form opening portions 48 and 49 for forming a cantilevered beam portion and a gap, respectively.
Then, the etching of the resulted substrate will be carried out by using the electrochemical etching stop technique.
In FIG. 4, there is shown an etching apparatus for use in the electrochemical etching. An etching bath 57 contains an alkaline etching solution 54 including hydrazine hydrate or KOH, and the resulted substrate 53 shown in FIG. 3e and a cathode 55 are immersed in the etching solution 54. The substrate 53 and the cathode 55 are connected to respective positive and negative electrodes of a power source 56 to effect the electrochemical etching. In this case, by utilizing the corrosive voltage difference between the p-type regions 41 and 44 and the n-type region 42, only the p-type regions can be selectively etched.
In FIG. 3f, the electrochemical etching is carried out in the lower surface of the substrate 41 using the lower silicon oxide film 47 as the mask, as described above, and the etching is stopped at the pn junction between the p-type substrate 41 and the n-type epitaxial layer 42 to selectively etch only the p-type regions 41 and 44. Hence, a cantilevered beam 50 and a weight 51 are formed, and a gap 52 of the etched p-type diffusion region 44 is arranged between the weight 51 and the substrate 41.
In FIG. 3g, the electrode 46 is removed to finish the semiconductor accelerometer shape processing. The upper and lower silicon oxide films 43 and 47 are removed.
In this method, since the pn junction is used as the etching stopper, the control of the thickness of the cantilevered beam can be readily achieved. However, the electrochemical etching method is used, and the electrode for applying the voltage during the electrochemical etching process is formed on the substrate. Further, the electrochemical etching is sensitive to the positional relation between the substrate and the cathode in the etching solution, and thus it is difficult to carry out the batch processing. Hence, in this case, an increase in cost is obtained.
In general, in order to improve the sensitivity of the accelerometer, the cross section of the cantilevered beam is preferably small within the limit of the strength allowed. In conventional semiconductor accelerometers, the cross section of the cantilevered beam is approximately a rectangular shape, and there is a problem in strength when the cantilevered beam is minimized.
In FIGS. 5a and 5b, there is shown a mounting structure of the semiconductor accelerometer shown in FIG. 1, including lower and upper stoppers 26 and 27 mounted to the lower and upper surfaces of the substrate 21 for preventing the cantilevered beam 22 from fracture due to an excessive acceleration to be applied to the beam 22 when, for example, dropping the accelerometer or the like.
However, such a conventional semiconductor accelerometer or mounting structure has the following problems.
Firstly, in the manufacturing process of the accelerometer, no protector for stopping the excessive displacement of the weight is provided after the formation of the cantilevered beam before the formation of the stoppers. Hence, the accelerometer must be handled carefully so as not to break the cantilevered beam, and productivity is largely lowered.
Secondly, the stopper forming process is complicated and costly. One of reasons why the accelerometer is formed from the semiconductor is to intend cost reduction per chip by fabricating many chips using a batch process, i.e., many chips are formed on a wafer and are processed in the same time to obtain the chips with stable quality and low cost. However, since the stoppers are attached to the upper and lower surfaces of the accelerometer after the formation of the cantilevered beam in the wafer process, as shown in FIGS. 5a and 5b, the advantage of the batch fabrication is lost and the cost increases largely.
Thirdly, it is difficult to form the stoppers accurately. To meet design requirements of the cantilevered beam, the gaps between the stoppers and the weight are controlled to be accurately formed as small as several .mu.m to several tens .mu.m in the structure shown in FIGS. 5a and 5b. The stoppers are required to be formed accurately and attached to the accelerometer precisely. Hence, high technology in preparing and bonding the stoppers cause the increase in cost.
Further, in the conventional accelerometer described above, an additional metal weight may be attached onto the silicon weight 23 in order to minimize the sensitivity of another axis, but the thickness of this additional metal weight tends to vary and to deteriorate the accuracy of the gap between the stoppers and the metal weight on the silicon weight. It is difficult to realize the stoppers with high accuracy effects.